The development of retinofugal and corticotectal connections, and the modifiability of these connections following either visual deprivation or surgical perturbation of the system will be examined in the rabbit using both anatomical and electrophysiological methods. In one set of experiments, the relative time of arrival of optic nerve fibers in the contralateral and ipsilateral dorsal lateral geniculate nucleus and superior colliculus will be studied using standard degeneration methods and a variety of recently developed antrograde pathway tracing techniques. The retinal location of the early-arriving contralateral fibers, and the initial distribution in the target nucleus of the ipsilateral retinal afferent terminals will be determined. Finally, after removal of one eye at various pre- and postnatal time , the amount of expansion of the ipsilateral projection from the remaining eye will be measured both anatomically and electrphysiologically. A second set of experiments will study the functional role of the descending corticocollicular pathway in the rabbit. The effect of inactivation of the descending cortiotectal pathway on the receptive field properties of cells in the adult rabbit superior colliculus will be examined directly using a cortical cooling technique. I will also study the development of the orientation-sensitive receptive fields in the superior colliculus following neonatal abation of visual cortex. Finally, the normal time of arrival of the corticofugal axons in the superior colliculus will be determined with anatomical methods. Expansion of this descending afferent input to the contralateral superior colliculus following neonatal removal of the contralateral visual cortex will be assessed. If the corticotectal pathway does show this expansion, then the functional effect of this aberrant pathway on the receptive field properties of cells in the affected superior colliculus will be investigated.